This invention relates to the conversion of radiation to other useful forms of energy and, more particularly, to a miniature radioactive light source and a method of its manufacture.
Miniature radioactive light sources are currently employed to backlight liquid crystal displays in digital watches and other instruments with visual displays. In contrast to incandescent lamps, a radioactive light source requires no electrical power source, and provides many years of maintenance free operation. Such a radioactive light source comprises a glass tube sealed at its ends, phosphor coated on its inner surface, and filled with tritium gas. When beta emission from the tritium strikes the phosphor coating, visible light is emitted.
The glass tube may have a circular or elongated cross section. An elongated cross section has the advantage that a larger area of a liquid crystal display can be illuminated by a single light source without increasing the thickness of the liquid crystal display-light source assembly. Further, a wide light source having an elongated cross section, makes more efficient use of the tritium gas.
The described miniature radioactive light sources are manufactured in the following way: the inner surface of a long glass tube is coated with a phosphor compound; the long, phosphor coated tube is filled with tritium and sealed at its ends with a gas flame; the long, tritium filled tube is subdivided into shorter tube segments by means of a laser beam to produce the light sources; and the resulting light sources are tested for radiation leakage.
Government licensing regulations place stringent requirements on the external radiation level of such radioactive light sources. If the light sources do not pass the leakage test, they must be rejected. Thus, reliable laser sealed ends on the glass tube are essential to good quality control in mass production.